Abstract

Environmental predictability is predicted to shape the evolution of life histories. Two key types of environmental predictability, seasonality and environmental colour, may influence life-history evolution independently but formal considerations of both and how they relate to life history are exceedingly rare.

Here, in a global biogeographical analysis of over 800 marine invertebrates, we explore the relationships between both forms of environmental predictability and three fundamental life-history traits: location of larval development (aplanktonic vs. planktonic), larval develop- mental mode (feeding vs. non-feeding) and offspring size.

We found that both dispersal potential and offspring size related to environmental predictability, but the relationships depended on both the environmental factor as well as the type of predictability. Environments that were more seasonal in food availability had a higher prevalence of species with a planktonic larval stage.

Future studies should consider both types of environmental predictability as each can strongly affect life-history evolution.

Abstract

Traditionally, it has been assumed that sperm are a vehicle for genes and nothing more. As such, the only source of variance in offspring phenotype via the paternal line has been genetic effects. More recently, however, it has been shown that the phenotype or environment of fathers can affect the phenotype of offspring, challenging traditional theory with implications for evolution, ecology and human in vitro fertilisation.

Here, I review sources of non-genetic variation in the sperm phenotype and evidence for co-variation between sperm and offspring phenotypes. I distinguish between two environmental sources of variation in sperm phenotype: the pre- release environment and the post-release environment.

Pre-release, sperm phenotypes can vary within species according to male phenotype (e.g. body size) and according to local conditions such as the threat of sperm competition. Post-release, the physicochemical conditions that sperm experience, either when freely spawned or when released into the female reproductive tract, can further filter or modify sperm phenotypes.

I find evidence that both pre- and post-release sperm environments can affect offspring phenotype; fertilisation is not a new beginning – rather, the experiences of sperm with the father and upon release can drive variation in the phenotype of the offspring.

Interestingly, there was some evidence for co-variation between the stress resistance of sperm and the stress resistance of offspring, though more studies are needed to determine whether such effects are widespread.

Overall, it appears that environmentally induced covariation between sperm and offspring phenotypes is non-negligible and further work is needed to determine their prevalence and strength.

Abstract

The relationship between offspring size and performance determines the optimal trade-off between producing many small offspring or fewer large offspring and the existence of this relationship has become a central tenet of life-history theory.

For organisms with multiple life-history stages, the relationship between offspring size and performance is the product of the effects of offspring size in each life-history stage.

Marine invertebrates have long been a model system for examining the evolutionary ecology of offspring size, and whilst offspring size effects have been found in several life-history stages, the crucial stage of colonization has received less attention.

We examined the effect of offspring size on the settlement response of sea-urchin larvae (Heliocidaris erythrogramma) to preferred and less preferred hostplants, how these effects changed over the larval period and estimated the success of juveniles in the field on preferred and less preferred host plants.

We found that smaller larvae became competent to respond to preferred host plant cues sooner than larger larvae but larger larvae rejected less preferred host plants for longer than smaller larvae. Overall, smaller H. erythrogramma larvae are likely to have less dispersal potential and are more likely to settle in less preferred habitats whereas larger larvae appear to have an obligately longer dispersal period but settle in preferred habitats.

Our results suggest that marine invertebrates that produce non-feeding larvae may have the potential to affect the dispersal of their offspring in previously unanticipated ways and that offspring size is subject to a complex web of selection across life-history stages.

Abstract

The optimal balance of reproductive effort between offspring size and number depends on the fitness of offspring size in a particular environment.

The variable environments offspring experience, both among and within life-history stages, are likely to alter the offspring size/fitness relationship and favor different offspring sizes. Hence, the many environments experienced throughout complex life-histories present mothers with a significant challenge to optimally allocate their reproductive effort.

In a marine annelid, we tested the relationship between egg size and performance across multiple life-history stages, including: fertilization, larval development, and post-metamorphosis survival and size in the field.

We found evidence of conflicting effects of egg size on performance: larger eggs had higher fertilization under sperm-limited conditions, were slightly faster to develop pre-feeding, and were larger post-metamorphosis; however, smaller eggs had higher fertilization when sperm was abundant, and faster planktonic development; and egg size did not affect post-metamorphic survival.

The results indicate that egg size effects are conflicting in H. diramphus depending on the environments within and among life-history stages. We suggest that offspring size in this species may be a compromise between the overall costs and benefits of egg sizes in each stage and that performance in any one stage is not maximized.

Published in: The American Naturalist, volume 184, number 2 (August 2014)

Abstract

Within and across taxa, there is much variation in the mode of fertilization, that is, whether eggs and/or sperm are released or kept inside or on the surface of the parent’s body.

Although the evolutionary consequences of fertilization mode are far reaching, transitions in the fertilization mode itself have largely escaped theoretical attention.

Here we develop the first evolutionary model of egg retention and release, which also considers transitions between hermaphroditism and dioecy as well as egg size evolution. We provide a unifying explanation for reported associations between small body size, hermaphroditism, and egg retention in marine invertebrates that have puzzled researchers for more than three decades.

Our model, by including sperm limitation, shows that all these patterns can arise as an evolutionary response to local competition between eggs for fertilization. This can provide a general explanation for three empirical patterns:

sperm casters tend to be smaller than related broadcast spawners,

hermaphroditism is disproportionately common in sperm casters, and

offspring of sperm casters are larger.

Local gamete competition also explains a universal sexual asymmetry: females of some species retain their gametes while males release theirs, but the opposite (“egg casting”) lacks evolutionary stability and is apparently not found in nature.

Abstract

Prior residence by a species can affect subsequent community assembly. However, previous studies insulated their focal communities from additional sources of variation, and the role of resident species in the context of environmental heterogeneity is rarely considered.

If environmental and resident species effects act independently, then each should be broadly predictable, and their contribution to community assembly should be quantifiable in relation to each other. Alternatively, if effects interact, their combination may explain more of the differences in communities than the additive influence of each alone.

We estimated the effects of a common, early-colonising resident (the encrusting bryozoan Hippopodina iririkiensis) on community assembly relative to substrate orientation. Some species showed complex responses in association with orientation, with positive responses in one orientation, negative in the other. Variation in orientation explained the majority of variation in overall community assembly. Variation among the panels holding replicates of our resident species, a blocking factor in the analysis, permitted us to consider small-scale spatial variation.

Abundances responded to resident species effects but interacted with spatial variation: the impact of the resident species on community assembly varied with orientation and space. Functional groups showed similarly idiosyncratic responses to the prior resident.

Overall, we found that resident species effects were weak relative to the effects of environmental variation on community assembly. Furthermore, those resident species effects that we did detect were inconsistent across environments, suggesting that this species has little predictable influence on community assembly.

Environmental variation may be an important contributor and requires more widespread consideration to better understand how resident species effects act in nature.

Published in: Proceedings of the Royal Society B, volume 281 number 1788 (July 2014)

Abstract

Metamorphosis is common in animals, yet the genetic associations between life cycle stages are poorly understood.

Given the radical changes that occur at metamorphosis, selection may differ before and after metamorphosis, and the extent that genetic associations between pre- and post-metamorphic traits constrain evolutionary change is a subject of considerable interest. In some instances, metamorphosis may allow the genetic decoupling of life cycle stages, whereas in others, metamorphosis could allow complementary responses to selection across the life cycle.

Using a diallel breeding design, we measured viability at four ontogenetic stages (embryo, larval, juvenile and adult viability), in the ascidian Ciona intestinalis and examined the orientation of additive genetic variation with respect to the metamorphic boundary.

We found support for one eigenvector of G (gobsmax), which contrasted larval viability against embryo viability and juvenile viability. Target matrix rotation confirmed that while gobsmax shows genetic associations can extend beyond metamorphosis, there is still considerable scope for decoupled phenotypic evolution.

Therefore, although genetic associations across metamorphosis could limit that range of phenotypes that are attainable, traits on either side of the metamorphic boundary are capable of some independent evolutionary change in response to the divergent conditions encountered during each life cycle stage.

Abstract

Anticipatory parental effects (APEs) occur when parents adjust the phenotype of their offspring to match the local environment, so as to increase the fitness of both parents and offspring.

APEs, as in the evolution of adaptive phenotypic plasticity more generally, are predicated on the idea that the parental environment is a reliable predictor of the offspring environment.

Most studies on APEs fail to explicitly consider environmental predictability so risk searching for APEs under circumstances where they are unlikely to occur. This failure is perhaps one of the major reasons for mixed evidence for APEs in a recent meta-analysis.

Here, we highlight some often overlooked assumptions in studies of APEs and provide a framework for identifying and testing APEs.

Our review highlights the importance of measuring environmental predictability, outlines the minimal requirements for experimental designs, explains the important differences between relative and absolute measures of offspring fitness, and highlights some potential issues in assigning components of offspring fitness to parental fitness.

Our recommendations should result in more targeted and effective tests of APEs.

Abstract

Transgenerational phenotypic plasticity is increasingly recognized as an important buffer of environmental change – many studies show that mothers alter the phenotype of their offspring so as to maximize their performance in their local environment. Fewer studies have examined the capacity of parents to alter the phenotype of their gametes to cope with environmental change. In organisms that shed their gametes externally, gametes are extremely vulnerable to local stresses and transgenerational plasticity in the phenotypes of gametes seems likely in this group.

In a marine tubeworm, Hydroides diramphus, we manipulated the salinity environment that mothers and fathers experienced before reproduction and then examined the phenotype of their gametes, as well as the performance of those gametes and the resultant larvae in different salinities.

We found strong evidence for gamete plasticity – both mothers and fathers adaptively adjust the phenotype of their gametes to maximize the performance of those gametes in the salinity regime experienced by their parents. Parents were quite flexible in the phenotype of gametes that they produced: they could switch the salinity tolerance of their gametes back and forth depending on their most recent experience.

Gamete plasticity was not without risks, however. We observed strong trade-offs in performance when gametes experienced an environment that did not match that of their parents. These effects of the parental environment persist for the duration of the larval phase such that larvae may not be able to disperse to environments that do not match their parents. Gamete plasticity may therefore represent an important source of phenotype–environment mismatches.

Gamete plasticity may represent an important mechanism for coping with environmental change and an important source of maternal and paternal effects in species with external fertilization. Studies that seek to predict the impacts of stresses that persist across generations (e.g. ocean acidification) should include parental exposures to the stress of interest.

Published in:The American Naturalist, volume 183, number 6 (June 2014)

Abstract

Growth rate is increasingly recognized as a key life-history trait that may affect fitness directly rather than evolve as a by-product of selection on size or age.

An ongoing challenge is to explain the abundant levels of phenotypic and genetic variation in growth rates often seen in natural populations, despite what is expected to be consistently strong selection on this trait. Such a paradox suggests limits to how contemporary growth rates evolve.

We explored limits arising from variation in selection, based on selection differentials for age-specific growth rates expressed under different ecological conditions. We present results from a field experiment that measured growth rates and reproductive output in wild individuals of a colonial marine invertebrate (Hippopodina iririkiensis), replicated within and across the natural range of succession in its local community.

Colony growth rates varied phenotypically throughout this range, but not all such variation was available for selection, nor was it always targeted by selection as expected.

While the maintenance of both phenotypic and genetic variation in growth rate is often attributed to costs of growing rapidly, our study highlights the potential for fluctuating selection pressures throughout the life history and across environments to play an important role in this process.

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We work on questions ranging from community ecology through to quantitative genetics.

Most of our work focuses on sessile marine invertebrates living in coastal systems — these organisms are extremely amenable to manipulation and can be tracked in the field for extended periods of time.

Some of us are interested in traditional marine ecology whereas others are evolutionary biologists who happen to work on marine invertebrates.